Hello -- The collateral blood flow, in those fortunate enough to be able to form them, function as arteries, not veins. They will develop in certain people and not others, and they are essentially new arteries, but not native ones, so looked at as a peculiar phenomenon. The important thing is that they do work. As to the effect on EF, it depends upon the amount of damage to the left ventricle already present, the course of the collaterals, etc. In other words they can sometimes have a dramatically salutory effect and improve cardiac output significantly. In others they do nourish the muscle but if there is too much scar tissue the improvement in actual LVEF can vary from a lot to a little. This will become more clear over time in any given patient. The more regular and progressive the cardiac rehab program, the better the outcome, as a rule.